Cisco Catalyst 2960 Series, Configuration Manual

Page: 331 / 2288

switch receives this BPDU, it decrements the received remaining hop count by one and propagates this value
as the remaining hop count in the BPDUs it generates. When the count reaches zero, the switch discards the
BPDU and ages the information held for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same throughout
the region, and the same values are propagated by the region designated ports at the boundary.
Boundary Ports
In the Cisco prestandard implementation, a boundary port connects an MST region to a single spanning-tree
region running RSTP, to a single spanning-tree region running PVST+ or rapid PVST+, or to another MST
region with a different MST configuration. A boundary port also connects to a LAN, the designated switch
of which is either a single spanning-tree switch or a switch with a different MST configuration.
There is no definition of a boundary port in the IEEE 802.1s standard. The IEEE 802.1Q-2002 standard
identifies two kinds of messages that a port can receive:
• internal (coming from the same region)
• external (coming from another region)
When a message is internal, the CIST part is received by the CIST, and each MST instance receives its
respective M-record.
When a message is external, it is received only by the CIST. If the CIST role is root or alternate, or if the
external BPDU is a topology change, it could have an impact on the MST instances.
An MST region includes both switches and LANs. A segment belongs to the region of its designated port.
Therefore, a port in a different region than the designated port for a segment is a boundary port. This definition
allows two ports internal to a region to share a segment with a port belonging to a different region, creating
the possibility of a port receiving both internal and external messages.
The primary change from the Cisco prestandard implementation is that a designated port is not defined as
boundary, unless it is running in an STP-compatible mode.
If there is a legacy STP switch on the segment, messages are always considered external. Note
The other change from the Cisco prestandard implementation is that the CIST regional root switch ID field
is now inserted where an RSTP or legacy IEEE 802.1Q switch has the sender switch ID. The whole region
performs like a single virtual switch by sending a consistent sender switch ID to neighboring switches. In this
example, switch C would receive a BPDU with the same consistent sender switch ID of root, whether or not
A or B is designated for the segment.
IEEE 802.1s Implementation
The Cisco implementation of the IEEE MST standard includes features required to meet the standard, as well
as some of the desirable prestandard functionality that is not yet incorporated into the published standard.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
249
Information About MSTP